In the medical field, various types of body-implantable leads are known and used. Cardiac pulse generators, in particular, use implanted leads to both sense cardiac function and deliver stimulation pulses. One type of commonly used implantable lead is an endocardial lead.
Endocardial leads are attached at their proximal end to an implantable pulse generator and at their distal end to the endocardium of a cardiac chamber. Often the lead assembly is inserted into the heart through a vein. The lead generally has an inner conductor covered by an insulative sheath.
The distal end of an endocardial lead may engage the endocardium by either an active fixation mechanism or a passive fixation mechanism. Passive fixation mechanisms, such as a time assembly, lodge or passively fix the lead to the heart. Active fixation mechanisms use a structure, such as a helix or hook, to engage into or actively fix themselves to the heart.
A sharpened helix has been found to provide a reasonably secure means for fixing the lead to the heart. An exposed sharpened helix may damage a vein, however, during introduction. Thus many active fixation leads have helixes which either retract into the lead body or are shielded during introduction. See for example, U.S. Pat. No. 4,972,848 of Di Domenico (helix shielded within lead body which may be extended to engage cardiac tissue); U.S. Pat. No. 5,003,992 of Holleman et al. (plunger through helix guards against damage to tissue by the helix and may be retracted to engage cardiac tissue) and U.S. Pat. No. 4,827,940 of Mayer et al. (soluble cover shields helix until positioned proximate fixation site.) Among the most preferred methods of shielding a helix is where the helix may be retracted within or extended from the lead body.
While preventing the helix from unintentionally engaging tissue is important, the helix itself cannot be too small. Specifically if the helix is too small it will not truly screw through tissue, but rather will only drill or stick into tissue like a needle, providing very little fixation. Thus the helix must be of a certain minimal diameter. Because the helix preferably is retractable within the lead body, the lead body distal end must also be of a certain minimal size.
Besides having the helix at the distal end, it is generally preferable to also locate an electrode at the distal end of the lead. In the design where the helix may retract within the lead body, a distal tip electrode permit cardiac sensing without having to first screw the helix into the tissue. Generally, however, it is preferable to minimize the surface area of the electrode. A small surface area increases the impedance of the electrode. This, in turn, reduces the current required for stimulating pulses. Smaller surface areas, however, may inhibit cardiac sensing. In addition, it is also often desirable to provide the capability for the electrode to deliver a drug (such as asteroid) in the vicinity of the electrode/tissue interface.
In short an active fixation lead has a variety of components having competing requirements. The distal end should not be too small so as to accommodate the helix, yet the electrode located at the distal end should not be too large. In addition is also preferable for the electrode to deliver a drug, such as asteroid.